This invention relates to a process for recovering hydrocarbons from tailings produced by a dilution centrifuging circuit within an oil sand operation employing the known hot water extraction process. It also encompasses the hydrocarbon froth product obtained by the practise of the process.
The hot water extraction process, used commercially to recover bitumen from the Alberta oil sands, involves the following series of steps:
(1) SLURRING, HEATING AND CONDITIONING THE OIL SAND IN A ROTATING DRUM IN WHICH IT IS MIXED WITH STEAM, HOT WATER AND CAUSTIC;
(2) DILUTING THE SLURRY WITH MORE HOT WATER AND INTRODUCING IT INTO A PRIMARY SEPARATION VESSEL WHERE IT IS RETAINED TO PERMIT BUOYANT BITUMEN PARTICLES TO RISE TO THE SURFACE AND FORM PRIMARY FROTH;
(3) WITHDRAWING A STREAM OF MIDDLINGS FROM THE MIDPOINT OF THE SEPARATION VESSEL AND SUBJECTING IT TO INDUCED AIR FLOTATION IN A SECONDARY RECOVERY FLOTATION CALL TO CAUSE CONTAINED BITUMEN TO FORM SECONDARY FROTH; AND
(4) COMBINING THE PRIMARY AND SECONDARY FROTHS TO FORM A COMBINED FROTH PRODUCT.
The dilution centrifuging process used to remove water and solids from the froth product of the hot water extraction process involves the following steps:
(1) DILUTING THE COMBINED FROTH PRODUCT WITH NAPHTHA TO ALTER THE VISCOSITY AND SPECIFIC GRAVITY OF THE CONTAINED BITUMEN; AND
(2) INTRODUCING THE DILUTED FROTH INTO A TWO-STAGE CENTRIFUGAL SEPARATION CIRCUIT WHERE THE COARSE SOLIDS ARE SEPARATED FROM THE STREAM IN A SCROLL-TYPE CENTRIFUGE AND THE WATER AND FINE SOLIDS ARE SEPARATED IN A DISC-TYPE CENTRIFUGE.
The combined hot water extraction and dilution centrifuging processes are hereinafter collectively termed "hot water extraction operation".
There is a significant loss of hydrocarbons in the dilution centrifuging (D.C.) tailings produced by the scroll and disc centrifuges. More particularly, in the plant presently being constructed by the assignee of this invention, there will be approximately 1.6 million pounds/hour of D.C. tailings produced containing approximately 3.5% by weight bitumen, 2.1% hydrocarbon diluent, 24.0% solids and 70.4% water. The losses of bitumen and diluent per day will be approximately 3,900 barrels and 3,200 barrels, respectively. These figures are only given by way of example, as it is expected that they will vary significantly with plant operating conditions and throughput.
The tailings composition shown above corresponds to the predicted values for a commercial plant. The tailings actually used in developing this invention were derived from pilot plant operations and contained a significantly greater proportion of water, as indicated in Example I. This extra water is a direct result of the mechanical configuration of the pilot plant centrifuges. However, the recovery of hydrocarbon from the pilot plant tailings is more difficult than it will be from the actual tailings, because the hydrocarbon content of the pilot plant tailings is reduced by the extra water.
A problem which has heretofore existed was how to economically recover the hydrocarbons contained in the D.C. tailings. If such a process could be developed, it would also be desirable that the diluent and bitumen be recovered in a single step. In this regard, it needs to be kept in mind that the D.C. tailings is a unique and difficult material to deal with. It contains a relatively small amount of bitumen and diluent distributed throughout a large quantity of water and solids. The bitumen in the tailings is originally recovered as part of the froth produced by the hot water extraction process. Its rejection to the D.C. tailings together with the proportionate amount of diluent suggests that these hydrocarbons are physically associated with the froth solids. Hence, despite the fact that the specific gravity of the diluted bitumen is less than 1.0, it still reports to the tailings. This is borne out by simple settling tests on D.C. tailings, in which it is found that only minor quantities of the hydrocarbons present float and the bulk of them are found in the solids which settle out.